A quantum key distribution system is configured with a transmitter, a receiver, and an optical fiber link that connects the transmitter and the receiver. The transmitter transmits photons to the receiver via the optical fiber (a quantum commination channel). Then, the transmitter and the receiver exchange control information in the form of digital data with each other; and share, in a confidential fashion, bit strings serving as cryptographic keys. This technology is implemented using the technology generally referred to as quantum key distribution (QKD).
A technology has been proposed by which such a quantum key distribution system is integrated into an optical-data communication network system that performs regular optical data communication. Herein, the technology that enables transmission and reception of photons for the purpose of quantum key distribution and enables optical data communication at the same time using the same optical fiber link is referred to as a “co-existence technology”. Thus, in the co-existence technology, a photon communication channel, which is used in transmitting and receiving photons, and an optical data communication channel, which is used in performing optical data communication, are implemented at the same time in the same optical fiber link. As a result, it becomes possible to reduce the cost of laying a new optical fiber required to implement the quantum key distribution system. Meanwhile, in the optical data communication channel mentioned above, not only the control information can be transferred but also the data not related to quantum key distribution can be transferred.
However, generally, the light used in the optical data communication channel has a strong optical intensity, while the light used in the photon communication channel has a weak optical intensity. For that reason, the light used in the optical data communication channel causes a noise for the photons in the photon communication channel. Because of such noise, the error rate in the photon communication channel goes on increasing, thereby making the operations of the quantum key distribution system unstable. Herein, the error rate is assumed to be the rate of occurrence of errors, from any cause, in the photon strings transmitted and received using the photon communication channel.